Such a current-to-voltage converter happens to have particular application in a high fidelity amplifier with high linearity.
It is common in such an amplifier, to use at the input, a digital-to-analogue converter such as the component PCM 1792 from the company Texas Instruments. This digital-to-analogue converter has a current output, such that the analogue signal is modulated in intensity.
Insofar as the amplification stage placed downstream uses at the input a modulated voltage, it is necessary to have a current-to-voltage converter between the digital-to-analogue converter and the actual amplifier stage itself.
The current output digital-to-analogue converters are particularly appreciated for the very low harmonic distortion level that they are able to reach. The difficulty consists in exploiting the qualities of such digital-to-analogue converters via a current-to-voltage conversion stage that does not in itself impact the performance of the digital-to-analogue converter.
The current sources contained in such a digital-to-analogue converter are connected to one or more outputs connected to a virtual ground or even connected between a ground and one or more outputs connected to a virtual ground. Such connections are conventionally implemented by means of an operational amplifier mounting arrangement.
A virtual ground is a fixed potential.
In this manner, all the transistors of the digital-to-analogue converter operate at constant current and voltage, therefore in an optimal fashion, regardless of the modulation of the output signal.
In order to preserve this absence of distortion, the current-to-voltage converters placed downstream may also, in a different implementation of this operational amplifier mounting arrangement, comprise specific stages, referred to as “common gate” stage based on MOSFETs (from the English term Metal Oxide Semiconductor Field Effect Transistor) or referred to as “common base” that is based on bipolar transistors. Such common base or common gate-based stages are operated in an open loop.
This type of arrangement comprising of common base or common gate-based transistors, commonly known as “cascode” is described in the document WO 2011/107671.
However, the transistors of such common base or common gate-based stages induce error currents which come to be superposed over the current originating from the digital-to-analogue converter, which is the root cause of the degradation of the starting digital signal.
The error currents are in particular due to the currents absorbed into the gates of the transistors of the common base or common gate-based stages.
In order to overcome this problem, the document WO 2011/107671 describes a current-to-voltage converter having the current reinjection means for reinjecting the current absorbed in the gates of transistors of common base or common gate-based stages.
However, such a current-to-voltage converter is unable, when it is fabricated in the form of an integrated circuit and not by means of discrete components, to compensate for the current absorbed in the substrate on which is formed the current-to-voltage converter.
The current absorbed in the substrate is in fact particularly difficult to measure due to the fact that no reference potential is defined at the level of this substrate.
Moreover, when the current-to-voltage converter is fabricated in the form of an integrated circuit, it comprises ESD (abbreviation for “Electrostatic Discharge”) protections, which are capable of protecting the components of the integrated circuit from possible electrostatic discharges.
However, such ESD protection elements are also responsible for an absorption of the current originating from the digital-to-analogue converter which come to be superposed over the previous losses.